The present invention relates to a color image forming apparatus of the type electrostatically forming latent images by scanning a photoconductive element with a laser beam modulated by color image signals and developing each latent image by a toner of particular color, e.g., yellow, magenta, cyan or black. More particularly, the present invention is concerned with a color image forming apparatus selectively operable in a full color image forming mode using three or four colors or in a monocolor image forming mode.
An analog color electrophotographic copier of the type projecting a document image onto a photoconductive element is disclosed in Japanese Patent Laid-Open Publication No. 229041/1985. This copier selects a particular process speed in each of a full color and monocolor copy modes. A monocolor (black-and-white) printer capable of changing the process speed for image formation when the scanning density of a laser beam is changed is proposed in Japanese Patent Laid-Open Publication No. 224780/1989. Further, implementations for forming an image by use of a plurality of laser beams is taught in Japanese Patent Publication Nos. 49941/1982, 33019/1985, 45065/1989, 43294, etc.
Generally, the amount of toner to deposit on a sheet is three times to four times greater in a full color copy mode than in a black-and-white or monocolor copy mode. Further, since many of color documents carry graphic patterns thereon, their images occupy about three times greater area than the images of black-and-white documents which are, in many cases, texts. It follows that far greater fixing energy is consumed in the full color copy mode than in the black-and-white (or monocolor) copy mode. In the light of this, the speed and temperature for fixation are so selected as to implement the fixation of a full color copy. This, however, wastes substantial part of the fixing energy and, therefore, substantial part of the electric power when it comes to the fixation of a black-and-white (or monocolor) copy. It is to be noted that the fixing speed for a black-and-white (or monocolor) copy may be increased up to a speed about twice to four times (depending on the toner) as high as the fixing speed for a full color copy. Another problem is that since a stack of documents made up of full color copies and black-and-white copies are often copied together, the range of fixing temperatures satisfying both of the full color and black-and-white (or monocolor) fixing conditions is extremely limited. As a result, the fixing temperature has to be controlled with high precision. Moreover, since the toner also has to have a thermal characteristic accommodating a broad range of fixing temperatures, the range of resinous materials available for the toner is limited. Hence, it is extremely difficult to implement an optimal toner color characteristic and an optimal charging characteristic at the same time from the durability, safe environment and cost standpoint.